This invention relates to recovery of dissolved molybdenum from spent mandrel dissolving acid solutions and, more particularly, to the rapid and efficient recovery of dissolved molybdenum from a spent solution of mixed sulfuric acid-nitric acid.
In the processing of tungsten filament coils and especially double-coiled filaments, a very fine tungsten wire is wrapped about a small molybdenum mandrel to form a first coil. The coiled tungsten and molybdenum mandrel are then wrapped about a second mandrel so that the tungsten is in the form of a coil which in turn is formed into a coil. The formed coiled-coil is then slipped off the largest mandrel, leaving the first molybdenum mandrel in place within the turns of the smaller tunsten coil. After cutting to size, this member is immersed in a warm solution of mixed sulfuric acid and nitric acid which dissolves the retained molybdenum mandrel leaving the coiled-coil of tungsten intact. The resulting coiled-coil of tungsten is then in a form suitable for use, such as a filament for an electric lamp.
The relative concentrations of the mixed sulfuric acid and nitric acid are subject to some variation and a mixed acid solution of sulfuric acid (normality of thirteen) and nitric acid (normality of seven) has been found to be very suitable. When this acid solution has sufficient molybdenum dissolved therein so that the molybdenum is present in an amount of from about 40 grams to about 75 grams per liter of acid solution, the rate of molybdenum dissolution becomes sufficiently slow that the acid is regarded as "spent".
In U.S. Pat. No. 3,963,823, dated June 15, 1976 is disclosed a method for recovering the molybdenum from the spent acid. In accordance with the teachings of this patent, ammonium hydroxide is added to a diluted spent acid solution to partially neutralize same and raise the pH thereof to a value of from about 1.5 to about 3. The solution is then maintained in a heated condition for a pedetermined time sufficient to precipitate therefrom substantially all of the dissolved molybdenum as ammonium molybdate complex. This precipitated ammonium molybdate is then recovered for further use. More specifically, after initial partial neutralization, the solution temperature preferably is maintained at from 70.degree. C. to 80.degree. C. with constant solution agitation for a period of at least about ten hours and preferably from ten hours to twenty hours in order to precipitate the dissolved molybdenum as ammonium molybdate complex.
In the initial practice of the method as set forth in U.S. Pat. No. 3,963,823, the spent acid was first diluted to an acid/water volume ratio of about 1:1, in order to ensure that solution boiling did not occur during the neutralization step. Ammonium hydroxide was then added to the diluted solution to achieve the desired pH, which typically was about 2.5. As the ammonium hydroxide was added, the degree of dilution of the solution increased accordingly. After the solution was partially neutralized, it was stirred for about twelve hours to complete the precipitation. In later commercial refinements of this process, the ammonium hydroxide was replaced with anhydrous ammonia and the spent acid was initially diluted to an acid-water volume ratio of 1:3. After the solution was partially neutralized to achieve a pH of about 2.9, it was stirred for an additional twelve hours to complete the precipitation.